1. Field of the Invention
The present invention relates to a digital disk player and, more particularly, to a digital disk player which reproduces a record by reading from a dual layer disk having two signal recording layers or from a single layer disk having only one signal recording layer, and in which servo gain is controlled when reading from a dual layer disk.
2. Description of the Related Art
There are two types of disks for use with digital video disk (DVD) players: a single layer disk having only one signal recording layer and a dual layer disk having two signal recording layers. FIG. 4A shows the structure of a single layer disk having a signal recording layer RL, a transparent plastic layer SR (substrate) and a protective layer PL for protecting the signal layer. FIG. 4B shows the structure of a dual layer disk, which includes a dual layer structure constructed by combining two disks having signal recording layers RL1 and RL2, and transparent plastic layers (substrates) SR1 and SR2, respectively, with a spacing layer SPL interposed therebetween. In FIGS. 4A and 4B, objective lenses LS are also illustrated.
Conventional techniques for reproduction from compact disks (CDs) can be used for reproduction from single layer disks, but not from dual layer disks, because these techniques were developed without considering dual layer disk technology.
Optical disk players such as CD players, laser disk players and DVD players have a focus servo controller for converging a laser beam on a signal recording surface of a disk, and a tracking servo controller for positioning the beam at the center of a selected track formed on the signal recording surface. The focus servo controller generates a focus error signal having a voltage value which varies according to the distance between a position of an objective lens and a particular in-focus position (the position of the objective lens when the focal point of the objective lens coincides with the signal recording surface), and controls the position of the objective lens with respect to the disk so that the error signal becomes zero. The tracking servo controller generates a tracking error signal according to a deviation of the beam from the track center, and controls the beam position so that the error signal becomes zero.
It would be convenient for users if digital disk players were designed to be able to reproduce data both from single layer disks and from dual layer disks. Current digital disk players do not perform this function. Such a digital disk player design must be able to identify a disk loaded in the digital disk player as being either a single layer disk or a dual layer disk because reproduction from single layer disks and dual layer disks is based on different standards.
One type of disk read by digital disk players has a dual layer structure constructed, as shown in FIG. 7, by combining two disks having signal recording layers RL1 and RL2, respectively formed on transparent substrates SR1 and SR2, with a spacing layer SPL interposed therebetween. An objective lens LS is provided for reproduction from the disk, as shown in FIG. 7. The reflectivity of such a dual layer disk is 25 to 40%, and the first signal recording layer RL1 and the second signal recording layer RL2 have different reflectivities. The reflectivity of the first signal recording layer is smaller than that of the second signal recording layer.
Due to the different reflectivities of the first and second signal recording layers of a dual layer disk, servo error signals (focus error signal, tracking error signal) respectively reproduced from the first and second signal recording layers RL1 and RL2 differ in amplitude from each other. If the servo gain of a digital disk player is controlled by being adapted for one of the signal recording layers, then servo control cannot be performed accurately during reproduction from the other signal recording layer.